Plant Kingdom Notes

Plant Kingdom

Introduction

• In the system proposed by Whittaker (1969), living organisms are classified into five kingdoms: Monera, Protista, Fungi, Animalia, and Plantae.
• This chapter focuses on the classification within Kingdom Plantae.
• The understanding of the plant kingdom has evolved over time.
• Fungi, Monera, and Protista members with cell walls are now excluded from Plantae.
• Cyanobacteria, also known as blue-green algae, are no longer classified as algae.
• The chapter covers Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms under Plantae.
• Classification within angiosperms is also discussed.

Classification Systems

• Early systems used superficial morphological characters like habit, color, number, and shape of leaves.
• These artificial systems were based on vegetative characters or the androecium structure (Linnaeus).
• Artificial systems separated closely related species due to their reliance on a few characteristics.
• They gave equal weightage to vegetative and sexual characteristics, which is not acceptable because vegetative characters are easily affected by the environment.
• Natural classification systems were developed based on natural affinities among organisms.
• These consider external and internal features like ultrastructure, anatomy, embryology, and phytochemistry.
• A natural classification for flowering plants was given by George Bentham and Joseph Dalton Hooker.
• Phylogenetic classification systems based on evolutionary relationships are currently acceptable.
• This assumes that organisms in the same taxa share a common ancestor.
• Information from various sources is used to resolve classification difficulties, especially when fossil evidence is lacking.
• Numerical Taxonomy, which uses computers, is based on all observable characteristics.
  • Numbers and codes are assigned to characters, and data is processed.
  • Each character is given equal importance, and hundreds of characters can be considered.
• Cytotaxonomy uses cytological information like chromosome number, structure, and behavior.
• Chemotaxonomy uses the chemical constituents of plants to resolve confusions.

Algae

• Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic organisms (both fresh water and marine).
• They are found in moist stones, soils, and wood, and in association with fungi (lichen) and animals.
• Algae vary in form and size, from colonial forms like Volvox to filamentous forms like Ulothrix and Spirogyra.
• Marine forms like kelps can form massive plant bodies.
• Algae reproduce by vegetative, asexual, and sexual methods.
  • Vegetative reproduction is by fragmentation, where each fragment develops into a thallus.
  • Asexual reproduction is by different types of spores, commonly zoospores, which are flagellated and motile.
  • Sexual reproduction involves the fusion of two gametes.
    • Isogamous: Gametes are flagellated and similar in size (Ulothrix) or non-flagellated but similar in size (Spirogyra).
    • Anisogamous: Gametes are dissimilar in size (Eudorina).
    • Oogamous: Fusion between a large, non-motile female gamete and a smaller, motile male gamete (Volvox, Fucus).
• Algae are useful to humans in various ways:
  • They carry out at least half of the total carbon dioxide fixation on Earth through photosynthesis.
  • Being photosynthetic, they increase the level of dissolved oxygen in their environment.
  • They are primary producers of energy-rich compounds, forming the basis of food cycles for aquatic animals.
  • Many species of Porphyra, Laminaria, and Sargassum are used as food.
  • Marine brown and red algae produce hydrocolloids like algin (brown algae) and carrageen (red algae), which are used commercially.
  • Agar, obtained from Gelidium and Gracilaria, is used to grow microbes and in ice-cream and jelly preparations.
  • Chlorella, a unicellular alga rich in proteins, is used as a food supplement, even by space travelers.
• Algae are divided into three main classes: Chlorophyceae, Phaeophyceae, and Rhodophyceae.

Chlorophyceae

• Commonly called green algae.
• The plant body can be unicellular, colonial, or filamentous.
• They are usually grass green due to the dominance of chlorophyll a and b.
• Pigments are localized in definite chloroplasts, which may be discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped.
• Most members have one or more storage bodies called pyrenoids located in the chloroplasts.
  • Pyrenoids contain protein and starch.
• Some algae may store food as oil droplets.
• Green algae have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
• Vegetative reproduction is by fragmentation.
• Asexual reproduction is by flagellated zoospores produced in zoosporangia.
• Sexual reproduction varies and may be isogamous, anisogamous, or oogamous.
• Common examples include Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.

Phaeophyceae

• Brown algae are primarily found in marine habitats.
• They vary in size and form, from simple branched filamentous forms (Ectocarpus) to profusely branched forms (kelps).
• Kelps may reach a height of 100 meters.
• They possess chlorophyll a, c, carotenoids, and xanthophylls.
• Their color varies from olive green to brown depending on the amount of fucoxanthin.
• Food is stored as complex carbohydrates like laminarin or mannitol.
• Vegetative cells have a cellulosic wall covered by a gelatinous coating of algin.
• The protoplast contains plastids, a centrally located vacuole, and a nucleus.
• The plant body is attached to the substratum by a holdfast.
• It has a stalk (stipe) and leaf-like photosynthetic organ (frond).
• Vegetative reproduction is by fragmentation.
• Asexual reproduction is by biflagellate zoospores that are pear-shaped with two unequal laterally attached flagella.
• Sexual reproduction may be isogamous, anisogamous, or oogamous.
• Union of gametes may occur in water or within the oogonium.
• Gametes are pyriform (pear-shaped) and bear two laterally attached flagella.
• Common examples include Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.

Rhodophyceae

• Red algae are called so due to the predominance of the red pigment, r-phycoerythrin.
• Most are marine and found in warmer areas.
• They occur in well-lighted regions and at great depths in oceans.
• Red thalli are mostly multicellular with complex body organization.
• Food is stored as floridean starch, similar to amylopectin and glycogen.
• Vegetative reproduction is by fragmentation.
• Asexual reproduction is by non-motile spores.
• Sexual reproduction is oogamous and accompanied by complex post-fertilization developments using non-motile gametes.
• Common members include Polysiphonia, Porphyra, Gracilaria, and Gelidium.

Algae Divisions - Main Characteristics

Bryophytes

• Bryophytes include mosses and liverworts found in moist, shaded areas in the hills.
• They are called amphibians of the plant kingdom because they live in soil but depend on water for sexual reproduction.
• They occur in damp, humid, and shaded localities and play an important role in plant succession on bare rocks/soil.
• The plant body of bryophytes is more differentiated than that of algae.
• It is thallus-like and prostrate or erect, attached to the substratum by unicellular or multicellular rhizoids.
• They lack true roots, stem, or leaves but may possess root-like, leaf-like, or stem-like structures.
• The main plant body is haploid and produces gametes (gametophyte).
• Sex organs are multicellular.
  • The male sex organ is the antheridium, producing biflagellate antherozoids.
  • The female sex organ is the archegonium, flask-shaped and producing a single egg.
• Antherozoids are released into water and fuse with the egg to produce the zygote.
• Zygotes do not undergo reduction division immediately but produce a multicellular sporophyte.
• The sporophyte is attached to the photosynthetic gametophyte and derives nourishment from it.
• Some cells of the sporophyte undergo meiosis to produce haploid spores.
• These spores germinate to produce gametophytes.
• Bryophytes have little economic importance, but some mosses provide food for herbaceous mammals, birds, and other animals.
• Species of Sphagnum provide peat, used as fuel and packing material due to its water-holding capacity.
• Mosses along with lichens colonize rocks, making the substrate suitable for higher plants.
• Mosses form dense mats on the soil, reducing the impact of falling rain and preventing soil erosion.
• Bryophytes are divided into liverworts and mosses.

Liverworts

• Grow in moist, shady habitats like banks of streams, marshy ground, damp soil, bark of trees, and deep in the woods.
• The plant body is thalloid (e.g., Marchantia).
• The thallus is dorsiventral and closely appressed to the substrate.
• Leafy members have tiny leaf-like appendages in two rows on stem-like structures.
• Asexual reproduction is by fragmentation of thalli or by gemmae.
  • Gemmae are green, multicellular, asexual buds in small receptacles called gemma cups on the thalli.
  • Gemmae detach from the parent body and germinate to form new individuals.
• During sexual reproduction, male and female sex organs are produced on the same or different thalli.
• The sporophyte is differentiated into a foot, seta, and capsule.
• Spores are produced within the capsule after meiosis and germinate to form free-living gametophytes.

Mosses

• The predominant stage of the life cycle is the gametophyte, consisting of two stages.
  • The protonema stage develops directly from a spore and is a creeping, green, branched, and filamentous stage.
  • The leafy stage develops from the secondary protonema as a lateral bud and consists of upright, slender axes bearing spirally arranged leaves.
• They are attached to the soil through multicellular and branched rhizoids, bearing the sex organs.
• Vegetative reproduction is by fragmentation and budding in the secondary protonema.
• In sexual reproduction, antheridia and archegonia are produced at the apex of the leafy shoots.
• After fertilization, the zygote develops into a sporophyte, consisting of a foot, seta, and capsule.
• The sporophyte in mosses is more elaborate than in liverworts.
• Spores are formed in the capsule after meiosis.
• Mosses have an elaborate mechanism of spore dispersal.
• Common examples include Funaria, Polytrichum, and Sphagnum.

Pteridophytes

• Pteridophytes include horsetails and ferns.
• They are used for medicinal purposes and as soil-binders and are grown as ornamentals.
• Evolutionarily, they are the first terrestrial plants to possess vascular tissues – xylem and phloem.
• Pteridophytes are found in cool, damp, shady places, though some flourish in sandy-soil conditions.
• In pteridophytes, the main plant body is a sporophyte, differentiated into true root, stem, and leaves.
• These organs possess well-differentiated vascular tissues.
• Leaves in pteridophyta are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
• Sporophytes bear sporangia subtended by leaf-like appendages called sporophylls.
• In some cases, sporophylls form distinct compact structures called strobili or cones (Selaginella, Equisetum).
• Sporangia produce spores by meiosis in spore mother cells.
• Spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus.
• Gametophytes require cool, damp, shady places to grow.
• The spread of pteridophytes is limited and restricted to narrow geographical regions due to specific requirements and the need for water for fertilization.
• Gametophytes bear male and female sex organs called antheridia and archegonia, respectively.
• Water is required for transfer of antherozoids from the antheridia to the mouth of the archegonium.
• Fusion of male gamete with the egg forms a zygote.
• Zygote produces a multicellular well-differentiated sporophyte, the dominant phase of pteridophytes.
• In most pteridophytes, all spores are of similar kinds (homosporous).
• Genera like Selaginella and Salvinia produce two kinds of spores (macro and microspores) and are known as heterosporous.
• Megaspores and microspores germinate and give rise to female and male gametophytes, respectively.
• Female gametophytes are retained on the parent sporophytes for variable periods.
• The development of zygotes into young embryos takes place within the female gametophytes.
• This event is a precursor to the seed habit, an important step in evolution.
• Pteridophytes are classified into four classes: Psilopsida (Psilotum); Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), and Pteropsida (Dryopteris, Pteris, Adiantum).

Gymnosperms

• Gymnosperms (gymnos: naked, sperma: seeds) are plants in which ovules are not enclosed by any ovary wall and remain exposed before and after fertilization.
• The seeds that develop after fertilization are not covered (naked).
• Gymnosperms include medium-sized trees or tall trees and shrubs.
• The giant redwood tree Sequoia is one of the tallest tree species.
• Roots are generally tap roots.
• Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialized roots called coralloid roots are associated with N_2-fixing cyanobacteria.
• Stems are unbranched (Cycas) or branched (Pinus, Cedrus).
• Leaves may be simple or compound.
• In Cycas, the pinnate leaves persist for a few years.
• Leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity, and wind.
• In conifers, needle-like leaves reduce the surface area.
• Their thick cuticle and sunken stomata also help reduce water loss.
• Gymnosperms are heterosporous, producing haploid microspores and megaspores.
• The two kinds of spores are produced within sporangia borne on sporophylls arranged spirally along an axis to form lax or compact strobili or cones.
• Strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
• Microspores develop into a male gametophytic generation, highly reduced and confined to a limited number of cells.
• This reduced gametophyte is called a pollen grain.
• The development of pollen grains takes place within the microsporangia.
• Cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
• Male or female cones or strobili may be borne on the same tree (Pinus).
• In Cycas, male cones and megasporophylls are borne on different trees.
• The megaspore mother cell is differentiated from one of the cells of the nucellus.
• The nucellus is protected by envelopes, and the composite structure is called an ovule.
• Ovules are borne on megasporophylls, which may be clustered to form the female cones.
• The megaspore mother cell divides meiotically to form four megaspores.
• One of the megaspores enclosed within the megasporangium develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs.
• The multicellular female gametophyte is also retained within the megasporangium.
• Unlike bryophytes and pteridophytes, in gymnosperms, the male and female gametophytes do not have an independent free-living existence.
• They remain within the sporangia retained on the sporophytes.
• Pollen grains are released from the microsporangium, carried in air currents, and come in contact with the opening of the ovules on megasporophylls.
• The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharges its contents near the mouth of the archegonia.
• Following fertilization, the zygote develops into an embryo, and the ovules into seeds.
• These seeds are not covered.

Angiosperms

• Unlike gymnosperms where ovules are naked, in angiosperms or flowering plants, pollen grains and ovules develop in specialized structures called flowers.
• In angiosperms, seeds are enclosed in fruits.
• Angiosperms are an exceptionally large group of plants occurring in a wide range of habitats.
• They range in size from the smallest Wolffia to tall trees of Eucalyptus (over 100 meters).
• They provide food, fodder, fuel, medicines, and other commercially important products.
• They are divided into two classes: the dicotyledons and the monocotyledons.